The present invention relates to a separation method of nitride semiconductor layer, a semiconductor device and a manufacturing method thereof.
The present invention also relates to a semiconductor wafer, and a manufacturing method thereof.
As an example of a conventional manufacturing method of a semiconductor device, Patent Document No. 1 discloses a manufacturing method of an LED (Light Emitting Diode), in which a buffer layer, an n-type nitride semiconductor layer, an active layer, a p-type nitride semiconductor layer, an n-type contact layer are grown on a sapphire substrate in this order so as to form a semiconductor wafer. Further, these layers are etched so as to partially remove the n-type contract layer, the p-type nitride semiconductor layer and the active layer, and to expose the n-type nitride semiconductor layer. Then, an Al (aluminum) electrode is formed on the exposed surface of the n-type nitride semiconductor layer and on the surface of the n-type contact layer using a deposition method. Thereafter, a heat treatment is performed to ensure a sufficient contact.
The growth of the above described layers (i.e., semiconductor epitaxial layers) is carried out using an MOCVD (Metal Organic Chemical Vapor Deposition) method. Using such a method, a nitride LED with high light emission efficiency is manufactured.
Recently, there is a need for integration of LEDs formed of different materials or integration of semiconductor devices with different functions. For this reason, it is preferred to separate the epitaxial semiconductor layers from the sapphire substrate (referred to as a first substrate), and to fix the semiconductor epitaxial layers to another substrate (referred to a second substrate).
Conventionally, there is proposed a laser lift-off method, in which a laser beam is irradiated onto a backside of the sapphire substrate to thereby decompose the nitride semiconductor layer in the vicinity of the sapphire substrate, by which the semiconductor epitaxial layers can be separated from the sapphire substrate. To be more specific, the laser beam irradiation onto the backside of the sapphire substrate causes GaN (gallium nitride) to be decomposed into Ga (gallium) and N (nitrogen) at an interface between the sapphire substrate and the nitride semiconductor layer. Since a melting point of Ga is at a room temperature, the semiconductor epitaxial layers can be separated from the sapphire substrate.
In this method, however, a separation surface of the semiconductor epitaxial layers separated from the sapphire substrate does not have a sufficient flatness (i.e., on the order of nanometers).
In order to join the nitride semiconductor epitaxial layers to a second substrate different from the first substrate (the sapphire substrate) using intermolecular force, it is preferred that the separation surface of the semiconductor epitaxial layers has a flatness on the order of nanometers. If the separation surface of the semiconductor epitaxial layers (separated from the first substrate) does not have a flatness on the order of nanometers, a sufficient intermolecular force (i.e., a joint force) is not obtained.
Therefore, in the method disclosed in Patent Document No. 1, it is necessary to perform a surface treatment to enhance the flatness of the separation surface of the semiconductor epitaxial layers, after the above described separation process using the lift-off method.
Meanwhile, a semiconductor crystal growth technology has been developed mainly for growing semiconductor layers on a lattice-matched substrate. Recently, a non-equilibrium epitaxial growth technology such as MBE (Molecular Beam Epitaxy) or MOCVD (Metal Organic Chemical Vapor Deposition) has been developed for growing semiconductor layers on a lattice mismatched substrate. In this regard, for example, Patent Document No. 2 discloses a manufacturing method of a nitride semiconductor device, in which a stack of an amorphous nitride layer and a GaN buffer layer is formed on a surface of a sapphire substrate, and then nitride semiconductor layers are grown on the stack.
However, in the method disclosed in Patent Document No. 2, there is a lattice mismatch of approximately 10% between the nitride semiconductor layers and the sapphire substrate. When crystal growth is performed while maintaining a covalent bonding with the substrate, crystal defects may be formed due to such a lattice mismatch at an interface. Therefore, there is a limit in enhancement of crystal characteristics of the grown nitride semiconductor layers.
In this regard, a high quality and large diameter nitride semiconductor wafer is still in the process of development. Therefore, there has been a demand for a crystal growth technology of high quality and large diameter single crystal nitride semiconductor on a lattice mismatched substrate.    Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2006-135311    Patent Document No. 2: Japanese Laid-Open Patent Publication No. H09-18053